Foam structures are known in industry and the number of applications for metallic foam structures is continually increasing. For example, aluminium foam metal having a continuously connected, open celled (reticulated) geometry is available and employed as energy and impact absorbers; in heat exchangers and as lightweight composite panels.
When used with heat exchanges the high surface to volume ratio allows for a compact design and the high specific stiffness, that is, high strength to weight ratio makes the material useful in aerospace and car applications.
Low-cost aluminium foam panels can be produced by a continuous casting process. The foam is machinable by common aluminium metal working techniques (sawing, drilling, milling) and may be joined by brazing or adhesive bonding. As previously indicated aluminium foam produced by this method finds application as lightweight cores for sandwich panels and as components in energy absorbing structures.
When irregular complex shapes are required, however, metal foams are formed typically by mixing small quantities of a gasifier e.g. titanium nitride with aluminium powder and subjecting the mixture to heat and pressure to form a sintered sheet.
The sintered sheet or a portion thereof is then placed in a mold which is then heated to a higher temperature at which the metal melts and nitrogen is released from the titanium nitride to provide an even dispersion of bubbles.
The hot metal is allowed to solidify and then is shock heat treated by dropping it into a cryogen such as liquid nitrogen which causes small fractures to occur between adjacent bubbles so that the mass becomes reticulated. This quenching process can be controlled by monitoring the temperature of the metal before it is quenched in the cryogen. The rate of cooling and the temperature difference, however, may still be insufficient to produce the necessary reticulated structure.
It is an aim of the present invention to add a further degree of control to the quenching process by employing the cryogen as a high velocity mixture of gas and liquid droplets.
According to the present invention a method of making a metal foam object includes the steps of mixing a gasifier with metal powder and subjecting the mixture to an elevated temperature T1 and pressure P1 to form a sintered sheet; placing at least a portion of the sintered sheet in a mold and subjecting the mold to a temperature T2 where T2 is greater than T1 at which the metal melts and the gas is released from the gasifier; and quenching the metal foam object thus formed by the mold in which the quenching is carried out by applying a cryogen to the object as a high velocity mixture of gas and liquid droplets.
Preferably the cryogen is nitrogen, the gasifier is titanium nitride and the metal is aluminium.